Polyphase coded fuzing system

ABSTRACT

A radar fuzing system for a guided missile is shown to include means for impressing a polyphase coded modulation on a transmitted signal and delayed replicas of such modulation on a bank of correlator/mixers, each one of the latter including dual gate field effect transistors as the active elements.

BACKGROUND OF THE INVENTION

This invention pertains generally to radar fuzing systems and moreparticularly to an improved mixer/correlator for use in such systems.

As is known in the art, radar fuzing systems, especially those designedfor use on board missiles where packaging volume is at a premium,operate at relatively low power levels. Therefore, such systems usuallyare operated in a continuous wave mode in order to maximize energy ontarget and are arranged to utilize some kind of binary coding technique(such as that known as polyphase coding) to obtain the requisite targetrange information. The most common type of polyphase coding is thatwherein the phase of the radio frequency (RF) carrier is alternatedbetween 0 and 180 degrees in accordance with an applied binary code. Therequisite phase switching is usually accomplished by so-called P-I-Ndiode phase shifters. Upon reception range information is obtained bycorrelation processing which is usually performed after the receivedsignals have been downconverted to intermediate frequency (IF) signalsor to baseband video signals. Such an approach generally implies areceiver with a relatively high noise figure.

Correlation of any polyphase modulated received signal is achieved bycomparing such signal with a delayed replica of the transmittedpolyphase code. The time delay corresponds to the desired range to beinstrumented at any particular time. The performance of any such system(meaning the accuracy with which range is measured and the degree towhich out-of-range clutter returns are rejected) is determined in thefirst instance by the by the accuracy with which the transmitted RFcarrier signal is modulated. Such modulation in turn is determined bythe switching speed of the diode phase shifters. Even though high speedP-I-N diodes may be used because of the relatively low power level ofthe RF carrier signal, the drive circuits for such diodes are usuallycomplex, are power consumptive, and require a substantial packagingvolume.

SUMMARY OF THE INVENTION

With this background of the invention in mind it is therefore an objectof this invention to provide an improved correlator/mixer capable ofperforming the combined functions of RF polyphase correlation anddownconversion to I.F. or baseband video frequencies.

It is another object of this invention to provide a correlator/mixerhaving improved bandwidth characteristics for correlating a widebandranging waveform.

Yet another object of this invention is to provide a correlator/mixerwherein the downconversion to I.F. or baseband video frequencies isaccomplished with a conversion gain.

These and other objects of the invention are attained generally byproviding a correlator/mixer including a pair of dual gate field effecttransistors (FETs). The source electrodes of each of the FETs areconnected together to a constant current source. The polyphase modulatedR.F. received signal is applied directly to one of the gate electrodesof a first one of the FETs and, via a 180 degree phase shifter, to oneof the gate electrodes of the second FET. A local oscillator (LO) signaland a replica of the transmitted code are applied to the second gateelectrode of each of the dual gate FETs. Because such coding signals arelow frequency signals in comparison with the LO signal, the formerappear as gate bias signals to the latter. A summing network is providedto combine the correlated and downconverted output signals appearing atthe drain electrodes of each of the FETs.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the invention, as well as theinvention itself, may be more fully understood from the followingdetailed description read together with the accompanying drawings, inwhich:

FIG. 1 is a simplified block diagram of a polyphase coded fuzing systemaccording to this invention; and

FIG. 2 is a schematic diagram, somewhat simplified, of thecorrelator/mixer shown generally in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a contemplated polyphase coded fuzing system 10is shown to include a pair of transmitting antennas 12 and a pair ofreceiving antennas 14. As is conventional, the transmitting antennas 12are mounted at diametrically opposite points on the body of a missile(not shown) and the receiving antennas 14 are mounted at points on thebody of the missile equally spaced between the transmitting antennas 12.The frequency of the R.F. carrier is controlled by a local oscillator16, the output signal of which is shown to be applied, via a circulator18, to a modulator 20. The latter is of conventional design and includesa pair of dual gate FETs for providing switching speeds in the range of70 picoseconds. Such a modulator is described in an article entitled"PSK and QPSK Modulators for Gigabit Data Rates" by C. L. Cuccia and E.W. Matthews, 1977 IEEE MTT-S International Microwave Symposium Digest.Suffice it to say here that in such a device the LO signal is split toprovide a pair of input signals separated in phase by 180 degrees. Suchphase opposed input signals are applied to the first gate electrodes ofseparate ones of the dual gate FETs and coding signals (T or T) from atiming generator 22 are applied to the second gate electrodes. The dualgate FETs are alternately switched "ON" to provide an R.F. signal withthe desired polyphase modulation.

The modulated R.F. signal from the modulator 20 is amplified in a threestage FET amplifier chain 24. The output signal from the latter is splitand applied as an input signal to a pair of output amplifiers 26, 28.The amplified output signals (which are, of course, the codedtransmitted signals) from the latter are passed, via circulators 30, 32,to the transmitting antennas 12.

Radar echo signals reflected from a target (not shown) are interceptedby the receiving antennas 14 and passed to a power combiner 34. Theoutput signals from the latter are limited by a limiter 36 prior tobeing split and applied to a bank of correlator/mixers 38₁, 38₂, 38₃,38₄ (an exemplary one of which is described in detail hereinbelow withreference to FIG. 2) along with delayed replicas of the codedtransmitted signals. Suffice it to say here that each one of thecorrelator/mixers 38₁, 38₂, 38₃, 38₄ is arranged to detect radar echosignals arriving at a different time after transmission of the codedtransmitted signal. With the delays for the correlation/mixers 38₁, 38₂,38₃, 38 ₄ properly set, the radar echo signals from a target will becorrelated in at least one of such correlator/mixers. The output signalsfrom the correlator/mixers 38₁, 38₂, 38₃, 38₄ are digitized byanalog-to-digital (A/D) converters 40₁, 40₂, 40₃ and 40₄ and then arepassed to a Fast Fourier Transform (FFT) signal processor 42. Thelatter, which is of conventional design, performs the functions ofDoppler processing and threshold detection of the correlated echosignals. The output signals from the FFT signal processor 42 are passedto a digital computer 44 wherein they are further processed to: (a)control, via the timing generator 22, the time delay of the codedsignals on the mixer/correlators 38₁, 38₂, 38₃ and 38₄ ; and (b), toprovide a firing command to the warhead (not shown) in the missile (notshown).

Referring now to FIG. 2, an exemplary one (mixer/correlator 38₁) isshown to include a pair of dual gate FETs Q1 and Q2. The sourceelectrodes (not numbered) of the dual gate FETs Q1 and Q2 are connected,via a constant current source (not numbered but comprising resistor R1capacitor 1 and dual gate FET Q3) to a negative bias supply -V_(ss).Phase coded signal R₁ from the timing generator 22 (FIG. 1) is appliedvia a coil 52 to a first gate electrode of FET Q1. An R.F. bypasscapacitor 54 is connected as shown and the LO signal from the circulator18 (FIG. 1) is connected (through a coupling capacitor 60) to a firstgate electrode of FET Q1. A load terminating resistor R4 is connected asshown. With the elements just described, the circulator 18 (FIG. 1) andthe timing generator 22 (FIG. 1) are effectively isolated one from theother. Similarly, a gate electrode of FET Q2 is connected as shown toreceive the R signal from the timing generator 22 (FIG. 1) and the LOsignal from the circulator 18 (FIG. 1). The elements in the circuitbeing discussed which are connected to FET Q2 are, as shown, coil 56, RFbypass capacitor 58, coupling capacitor 62 and resistor R5. The finaleffect, then, is that the FETs Q1, Q2 are alternatively turned "ON" inaccordance with the coding signals R1, R1. That is to say, coding signalR1 causes FET Q1 to be "ON" and coding signal R1 causes FET Q2 to be"ON".

The RF input signal from limiter 36 (FIG. 1) is shown to be split. Afirst portion of such RF input signal is applied, via a couplingcapacitor 64, to the second gate electrode of FET Q1. A second portionof such RF input signal is shifted by 180 degrees in a phase shifter 66and then is applied, via a coupling capacitor 68, to the second gateelectrode of FET Q2. The coupling capacitors 64, 68 are required toisolate the gate bias voltage, -V_(gg), from the limiter 36 (FIG. 1).Chokes comprising coils 88, 90 and capacitor 92 are provided to preventthe RF signals from limiter 38 (FIG. 1) from entering the gate biassupply -V_(gg). The gate bias voltage -V_(gg) is required to preventdamage to the FETs Q1 and Q2 caused by a possible excess drain currentduring turn-on and to provide the optimum operating point for gain andthe noise figure considerations. The drain electrodes (not numbered) ofFETs Q1 and Q2 are connected via coils 70, 74 and resistors R2, R3,respectively, to a bias supply VDD. The value of resistors R2, R3 ischosen to control the conversion gain of the correlator/mixer 38₁. RFbypass capacitors 72, 76 isolate the bias supply VDD.

In operation, when the coding signal R1 causes the FET Q1 to be "ON":(a) the then existing RF signal from the limiter 38 (FIG. 1) is feddirectly to an active element, i.e. an element (FET Q1) in whichcorrelation and downconversion may be accomplished; and (b) the thenexisting RF signal is first shifted in phase by 180° and then fed(without effect) to an inactive element FET Q2), i.e. an element inwhich correlation and downconversion cannot be accomplished. Conversely,when the coding signal R1 causes the FET Q2 to be ON: (a) the thenexisting RF signal out of the limiter 38 (FIG. 1) to be shifted in phaseby 180° and applied to an active element (FET Q2); and (b) the thenexisting RF signal out of the limiter 38 (FIG. 1) to be fed directly,but without effect, to an inactive element (FET Q1). The effect then isthat, at the selected time after the polyphase code is impressed on thesignals out of the modulator 20 (FIG. 1), a replica of such code isimpressed on the correlator/mixer 38₁. The nature of the then existingRF signal out of the limiter 36 (FIG. 1) then determines the nature ofthe outputs of FETs Q1, Q2. If the RF signals are echo signals from apoint source at the proper range, i.e. if the FETs Q1, Q2 are ONalternately to coincide with the coded echo signal from a target, theneach one of the FETS Q1, Q2 correlates with and mixes a part of thecoded echo signals with a corresponding part of the coded LO signals toremove the modulation applied in the modulator 20 (FIG. 1). On the otherhand, if the then existing RF signals from the limiter 36 (FIG. 1) areecho signals from extended targets (clutter) or are noise-like signals(no returns) correlation will not be achieved with the coded LO signalsso the mixing process results in signals out of the FETs Q1, Q2 whichare not coherent CW waveforms and therefore do not integrate coherentlyin the FET signal processor 42 to cause a valid detection (FIG. 1). TheR1 and R1 signals are complementary logic signals whose state determineswhich one of FETs Q1, Q2 is ON. If Q2 is ON a differential phase shiftof 180 degrees is applied to the RF carrier signal with respect to thatapplied if Q1 is ON. Thus, in the correlation mode the operation ofcorrelator/mixer 38₁ is identical to that of modulator 20 (FIG. 2).

The principles governing the use of a dual gate FET as a mixer havingconversion gain has been described in an article entitled "GaAs FETsGain Ground in Oscillators, Mixers, and Limiters", MICROWAVES, HaydenPublishing Co., Inc., Rochelle Park, New Jersey, June 1977, pp. 9-12.The downconversion mixer described therein employed a single dual gateFET having an LO signal and an RF carrier signal applied to differentones of the two gate electrodes, the source electrode grounded, an IFoutput signal at the drain electrode and a high pass, low pass filternetwork and an IF matching network connected to the drain electrode.

In the herein contemplated correlator/mixer 38₁ using two FETs in thedual gate configuration it has been found to be advantageous to connectthe source electrodes of the FETs Q1 and Q2 to the constant currentsource (not numbered) in order to match the drain currents in each ofFETs Q1 and Q2, thereby preventing the induced feedthrough of the R1, R1signals into the output circuit because of unequal voltage drops acrossresistors R2 and R3. Coil 70 and RF bypass capacitor 72 are provided onthe drain terminal of FET Q1 to prevent the leakage of the IF outputsignal from that device to the drain bias supply V_(DD). A similar coil74 and RF bypass capacitor 76 are provided on the drain terminal of FETQ2.

The output signals from FETs Q1 and Q2 are passed, via filters 78, 80and matching networks 82, 84, respectively, to a summing network 86wherein they are combined to provide output signals to be processedfurther. The filters 78, 80 prevent undesired frequencies produced inthe mixing process from being passed to the output circuit (notnumbered). The matching networks 82, 84 are provided to match therelatively high output impedance of FETs Q1 and Q2 to the 50 ohmimpedance of the summing network 86. It is felt that the design offilters 78, 80 and matching networks 82, 84 are matters involvingordinary skill in the art and will therefore not be recounted here.

It is noted here in passing that FETs Q1, Q2 and Q3 are here Model NEC463 devices from California Eastern Labs. I Edward Ct., Burlingame,California 94010, which have rise and fall times of 70 picoseconds orless and which represent approximately a fifty-to-one improvement inswitching speed over P-I-N diodes. The gate electrodes of FETs Q1 and Q2present no load to the drivers (not shown) other than the small gateparasitic capacitance and, therefore, FETs Q1 and Q2 require a gatevoltage change of only 3 o 4 volts to turn ON or OFF as opposed to 15volts or more for P-I-N diodes. Thus, it can be seen that the use ofFETs significantly simplifies the requisite drivers.

Having described a preferred embodiment of a correlator/mixerparticularly suited for correlating CW ranging waveforms, it will now beapparent to one of skill in the art that changes may be made withoutdeparting from the inventive concepts of this invention. For example, ifan IF output signal were desired then a separate local oscillator offsetin frequency from that in the transmitter channel could be used with thecorrelation mixers. If is felt, therefore, that this invention shouldnot be restricted to its disclosed embodiment, but rather should belimited only by the spirit and scope of the appended claims.

What is claimed is:
 1. In a radar fuzing system for use in a guidedmissile, such system being adapted periodically to transmit a radiofrequency signal modulated in accordance with a predetermined polyphasecode and, after reception of an echo signal, to process such signal in abank of similar correlator/mixers, each one of such correlator/mixerscomprising:(a) first and second dual gate field effect transistors; (b)means, operative during selected successive intervals of time betweenthe transmission of each predetermined polyphase code, for alternatelybiasing the first and second dual gate field effect transistors into aconducting state; (c) means for applying a local oscillator signal toone of the gates of each one of the first and the second dual gate fieldeffect transistors; (d) means for applying the echo signal directly tothe second gate of one of the dual gate field effect transistors and forreversing the phase of such signal and applying the resulting signal tothe second gate of the second one of such transistors; and (e) means forcombining the output signals from the dual gate field effecttransistors.
 2. A correlator/mixer as in claim 1 wherein the frequencyof the local oscillator signal is the same as the frequency of thetransmitted radio frequency signal.